1. Field of the Invention
This invention related to the field of integrated circuit structures. More particularly, this invention relates to an improved polysilicon/tungsten silicide multilayer composite formed on an integrated circuit structure, and a method of making same.
2. Description of the Related Art
The use of a layer of tungsten silicide over polysilicon is well known in the formation of integrated circuit structures. Conventionally, the respective polysilicon and tungsten silicide deposition steps were carried out in separate vacuum chambers with other intervening steps carried out outside of a vacuum environment. More recently, however, polysilicon and tungsten silicide depositions have been carried out in multiple vacuum chamber apparatus wherein the substrate is moved under vacuum from the polysilicon deposition chamber to the tungsten silicide deposition chamber. Such a procedure is described by Beinglass and Chang in "Integrating Polysilicon and Tungsten Silicide Deposition", published in Electronics Engineering in March 1994, at pages 108-109 and 112-113. This article describes the successful deposition of a polysilicon layer on a semiconductor substrate in one vacuum deposition chamber followed by transfer of the polysilicon-coated substrate through an intermediate transfer chamber to a second vacuum deposition chamber in the same apparatus for the deposition of tungsten silicide over the polysilicon layer without, however, exposing the coated substrate to ambient conditions in between the two depositions.
The deposition of the tungsten silicide (WSi.sub.x) layer, in the process described in the above article, was carried out using WF.sub.6 gas as the source of tungsten and silane (SiH.sub.4) gas as the source of silicon. However, it has been found that for certain applications, such as improved step coverage and reduced fluorine content in the deposited tungsten silicide, the use of dichlorosilane (DCS) as the source of silicon is preferable to the use of silane in the formation of tungsten silicide. By use of the term "improved step coverage" is meant, that the portion of the tungsten silicide coating lying over a polysilicon layer in a via or contact opening will have a thickness which is over 50% of the thickness of the tungsten silicide layer lying over the remainder of the substrate when the aspect ratio of the via or contact opening is 2:1. That is, referring to FIG. 1, the thickness "X" of tungsten silicide layer 8 over polysilicon layer 7 in via 4 formed in, for example, insulator layer 6 over integrated circuit structure 2 is over 50% of the thickness "Y" of tungsten silicide layer 8 over polysilicon layer 7 on the surface of layer 6. By use of the term "reduced fluorine content" is meant a fluorine concentration of less than 10.sup.18 fluorine atoms/cm.sup.3.
However, the use of DCS to form tungsten silicide in a multiple chamber vacuum system where the polysilicon-deposited surface is not subjected to a wash prior to deposition of the WSi.sub.x has given rise to a different problem, wherein the underlying doped polysilicon layer apparently acts as a nucleation layer for the deposition of a tungsten-rich tungsten silicide layer when DCS is the source of silicon. When an undoped layer of polysilicon is substituted for the doped layer, the problem does not occur. The term "tungsten-rich tungsten silicide" may be defined as a tungsten silicide layer wherein the tungsten to silicon ratio of the layer is not uniform, with the tungsten to silicon ratio in the tungsten silicide layer being higher adjacent the underlying silicon layer, which can, in turn, result in higher stress. Typically a "tungsten-rich tungsten silicide" will have a tungsten to silicon ratio of &gt;1:2.1 at the tungsten silicide/polysilicon interface, i.e., more than 1 tungsten atom per 2.1 silicon atoms, where the normal tungsten: silicon ratio used, having the formula WSi.sub.x, is such that x ranges from about 2.1 to about 2.9 throughout the tungsten silicide layer.
While a layer of undoped polysilicon could be formed over the integrated circuit structure on the substrate, instead of a doped layer, with the undoped polysilicon layer subsequently doped by an implantation step, after the deposition of the tungsten silicide layer, such an implantation step is sometimes not compatible with other portions of the integrated circuit substrate, nor is it economical, even if technically feasible.
It would, therefore, be desirable, to provide a structure and method wherein tungsten silicide may be formed over polysilicon, using DCS as a source of silicon, while avoiding the prior art problems which resulted from the use of DCS in such depositions, and without the need for a subsequent implantation step to dope the underlying polysilicon layer.